1. Field of the Invention
The present invention relates to a method of and a device for calculating the strength of an electromagnetic field radiated from an electric circuit apparatus using a moment method, and a storage medium for storing a program that simulates the device for calculating electromagnetic field strength. In particular, the present invention relates to a method of and a device for accurately constructing a model of a multilayer printed board installed in an electric circuit apparatus, and according to the model, correctly calculating the strength of an electromagnetic field radiated from the electric circuit apparatus, as well as a storage medium for storing a program that simulates the device for calculating electromagnetic field strength.
Many countries have regulations to restrict radiation of unnecessary radio waves and noise exceeding a predetermined level from electric circuit apparatuses.
To meet such regulations, shielding and filtering techniques are employed, and to quantitatively evaluate the radiation reducing effect of such shielding and filtering techniques, simulation techniques are needed.
The inventors of the present invention have developed simulation techniques for calculating the strength of an electromagnetic field radiated from an electric circuit apparatus according to the moment method. These techniques need an accurate model of a target electric circuit apparatus.
2. Description of the Related Art
When simulating the strength of an electromagnetic field radiated from an object, the electric and magnetic currents flowing through each part of the object must be found and substituted into known theoretical equations related to electromagnetic wave radiation. The electric and magnetic currents running through an object are theoretically obtained by solving Maxwell's electromagnetic wave equation under given boundary conditions.
The moment method is a technique to solve Maxwell's equation and provides one solution to an integral equation derived from Maxwell's equation. The moment method divides an object into small elements and calculates electric and magnetic currents flowing through the elements. The moment method is capable of handling a three-dimensional object having an arbitrary shape. The moment method is described in, for example, "Sinusoidal Reaction Formulation for Radiation and Scattering from Conducting Surface" by H. N. Wang, J. H. Richmond, and M. C. Gilreath, IEEE Transactions Antennas Propagation, Vol. AP-23, 1975.
The moment method converts the structure of a target electric circuit apparatus into a mesh of elements. After selecting a target frequency, the method calculates mutual impedance, mutual admittance, and mutual reaction among the elements, substitutes the calculated values and a wave source specified by structural data into simultaneous equations of the moment method, and solves the simultaneous equations, to find the electric and magnetic currents running through the elements.
When handling a metal object, the moment method divides the object into a mesh of elements, calculates mutual impedance Zij among the elements, and solves the following simultaneous equations involving the mutual impedance Zij, wave sources Vi, and currents Ii flowing through the metal elements: EQU Zij!Ii!=Vi!
Solving the simultaneous equations provides the currents Ii, which are used to calculate the strength of an electromagnetic field. In the above expression, "!" indicates a matrix.
If each element of the mesh involves resistance, capacitance, and reactance, they are added to the self-impedance of the element.
For realizing high-density mounting, electric circuit apparatuses frequently employ a multilayer printed board, which is composed of a power source layer, a ground layer, and a signal layer that are laminated one upon another with insulating material being interposed between each pair of the adjacent layers.
FIG. 15A shows an example of the multilayer printed board. More precisely, the example consists of a first signal layer 20, a first core 21 made of, for example, glass epoxy resin, a first prepreg 22 made of insulating material for thickness adjusting and bonding purposes, a power source layer 23, a second core 24 made of, for example, glass epoxy resin, a ground layer 25, a second prepreg 26 made of insulating material for thickness adjusting and bonding purposes, a third core 27 made of, for example, glass epoxy resin, and a second signal layer 28. There are four metal layers 20, 23, 25, and 28 and five insulating layers 21, 22, 24, 26 and 27.
FIG. 15B shows circuit parts and chips mounted along circuit patterns on the first signal layer 20 or on the second signal layer 28, to form an electronic circuit. The circuit patterns are made of a metal, such as copper, foil. The electronic circuit is electrically connected to the power source layer 23 and ground layer 25 through vias or holes, so that the electronic circuit may be supplied with a power and be grounded.
The electronic circuit formed on each signal layer (20, 28) radiates a strong electromagnetic field.
When simulating a target electric circuit apparatus having such a multilayer printed board, the prior art ignores the dielectric existing between the power source layer and the ground layer of the multilayer printed board and replaces the power source and ground layers with metal layers having no thickness, thereby modeling the multilayer printed board.
An electronic circuit mounted on each signal layer, however, produces noise when it operates. The noise causes a high-frequency current to flow to the power source and ground layers. As a result, the dielectric existing between the power source and ground layers resonates and works as a microstrip antenna to radiate an electromagnetic field that cannot be ignored.
In FIG. 15A, the power source layer 23 supplies power to the first and second signal layers 20 and 28 through vias, and the ground layer 25 provides the signal layers 20 and 28 with ground potential through vias. As a result, a noise source is present between the layers 23 and 25 as shown in FIG. 16 depending on the operation of the electronic circuit.
The noise source causes a high-frequency current to flow between the power source layer 23 and the ground layer 25 as shown in FIG. 17. As a result, the dielectric existing between the layers 23 and 25 resonates, working as a microstrip antenna to radiate an electromagnetic field that is strong and cannot be ignored.
Due to a speedup in electronic circuits such as an MPU mounted on signal layers of a multilayer printed board, high-frequency currents running between power source and ground layers of the board cannot be ignored. This is because the currents cause a dielectric between the power source and ground layers to resonate and produce a strong electromagnetic field.
In spite of this, the prior art ignores the dielectric between the power source layer and the ground layer when modeling the multilayer printed board and replaces the power source and ground layers with metal plates having no thickness. Accordingly, it is impossible for the prior art to calculate the strength of an electromagnetic field radiated from the multilayer printed board due to the resonance of the dielectric.